A chronograph is used to measure the average velocity of projectiles in a batch of test projectiles and also to indicate the dispersion of the velocity readings of the sample. It is commonly assumed that any measurement errors, i.e., errors introduced by the measurement system itself, are negligible compared to both the average velocity and to the dispersion. While the first assumption is usually justified, the second may not be.
Chronographs of the type under consideration herein are based on the well-known relationship that velocity of a projectile is equal to the distance traveled by the projectile divided by the time required to travel that distance. Systems such as those shown in U.S. Pat. Nos. 3,824,463; 4,128,761; and 4,239,962, all to the present inventor, illustrate devices for this purpose as they have been used in the past. The content of these patents is hereby incorporated by reference. As will be recognized from these patents, optical devices are preferred as the sensors for determining when the projectile passes through specific planes although inductive devices can be employed. The sensors, commonly referred to as "screens", are usually placed a few feet apart, perhaps ten feet, and the light perturbations detected by the photodetectors in these devices are supplied to timing circuits which respond to the interval between the pulses, and to a setting representative of the distance, to produce an output which quite accurately represents the velocity.
It is, of course, possible for errors to creep into the system. Some test procedures frequently require that two independent chronograph systems be used on each shot. One chronograph is commonly designated as "the primary" channel and the other is designated "secondary". The secondary chronograph verifies proper functioning of the primary system. Most chronograph errors are not errors in the time measuring circuits of the actual chronograph itself, but are usually errors contributed by the detecting devices (coils or screens) used to signal the passage of the projectile through the plane. If the two chronographs are independent, and if the detecting devices are physically spaced apart, it is very unlikely that the two velocity readings will agree when there is an error contributed by one or more of the detecting devices.
However, providing two entire systems is obviously a rather expensive and cumbersome solution to the problem. A major inconvenience is that the two systems, being independent, produce their own outputs which must then be manually compared in order to determine whether a problem exists. As a practical matter, this is generally done only after the entire shot sequence is completed and, if it turns out that there were errors, the statistical analysis of the results must be recomputed. The alternative would be to provide a supervisory system which receives the outputs of both independent systems but this, again, involves considerable cost and simply makes the operation more complex, introducing additional possibilities for error.